1. Field of the Invention
The present invention relates to a zoom lens barrel, and in particular, it relates to a multi-stage (in particular three-stage) extension type zoom lens barrel and a helicoid extension type zoom lens barrel.
2. Description of the Related Art
In conventional zoom compact cameras, it is preferable that the thickness (length) of the camera when not in use be at a minimum. To this end, a multi-stage extension type zoom lens barrel which has three stages has been employed. In the conventional three-stage extension type zoom lens barrel, a feed mechanism which transmits the drive force of a zoom motor to each lens barrel member is complex; requires maintenance; and makes the camera heavy and large. For example, in the conventional three-stage extension type zoom lens barrel, a second rotational barrel is provided in a second feed stage so as to move to in the direction of the optical axis with respect to a first feed stage upon rotation thereof. As means for to achieve such movement of the second rotational barrel, a helicoid engagement mechanism is well known. However, it is difficult to feed barrel members engaged by helicoids by only applying force in the direction of axis. Accordingly, if the second rotational barrel engages with a first rotational barrel of the first feed stage by means of helicoids, there will be need to provide a rotational transmission mechanism that transmits the rotational force of the first rotational barrel to the second rotational barrel. Such a rotational transmission mechanism is the major cause of making the feed mechanism of the zoom lens barrel complex. Of cause, this problem resides not only in three-stage extension type feed mechanisms but also in any multi-stage extension zoom lens barrel which needs to provide a rotational transmission mechanism between the feed stages.
A cam extension type rotation-feed mechanism for a zoom lens barrel is also known, in which a pair of barrel members are respectively provided on the opposed peripheral surfaces with a spiral lead groove (cam groove) and a projection which engages in the cam groove to extend the lens barrel. In the cam extension type, however, since the length of engagement between the barrel members is small, the rigidity of the lens barrel tends to be insufficient.
In general, in a multi-stage extension type zoom lens barrel, upon zooming, an inner barrel (cylinder) is moved or advanced relative to an outer barrel (cylinder) whose diameter is larger than that of the inner barrel. In this state, the inner barrel can be seen externally. Therefore, in order to prevent an external helicoid (male helicoid) provided on the outer peripheral surface of the inner barrel from being exposed to the outside, the male helicoid extends in the axial direction only by a length necessary to provide the maximum extension of the inner barrel, while an internal helicoid (female helicoid) provided on the inner peripheral surface of the outer barrel extends in the whole range of the movement. Consequently, the thickness of the portion of the inner barrel on which no external helicoid is formed is smaller than that of the remaining portion of the inner barrel, thus resulting in a reduced strength of the lens barrel. Moreover, if the range of the engagement between the external helicoid and the internal helicoid is small, there is a fear that light enters a space between the outer and inner barrels.
If it is accepted that the external helicoid formed on the inner barrel can be seen from outside, it is possible to provide the external helicoid on the entire peripheral surface of the inner barrel. However, if the external helicoid formed on the entire peripheral surface of the inner barrel engages with the internal helicoid formed on the entire peripheral surface of the outer barrel, the following problems arise. In general, the barrel components of a compact camera are made of a plastic mold or the like, and hence it is difficult to produce the barrel components accurately (e.g., roundness, etc.). Consequently, if the length of the engagement of the external and internal helicoids is long, the movement of the barrel member(s) interfere with the helicoids during the extension or retraction of the barrel member(s) due to a dimensional error, etc., thus resulting in a non-smooth movement of the barrel member(s). Namely, in the conventional helicoid extension type zoom lens barrels, it has been difficult to achieve sufficient strength and effective light interception together with smooth movement of the barrel members.